Wednesday, August 1, 2012

New paper finds Antarctica snow pack will continue to grow during 21st century

A paper published in the current issue of Nature Climate Change predicts that increases in precipitation over Antarctica in the 21st century will act as a negative feedback and overcompensate for surface warming, leading to a net increase in snow pack/albedo over the 21st century. According to the authors, "We deduce that projected future increases in precipitation4 can increase snow albedo by 0.4% on average during the twenty-first century and hence overcompensate the expected albedo decrease owing to warming (0.3% for 3°C)." Antarctica is home to 90% of the world's ice.

The high albedo of snow largely determines the climate of polar regions by controlling energy absorption at the surface. In Antarctica, where light-absorbing impurities are few, snow albedo is mostly determined by the size of snow grains1. Snow metamorphism, the process of grain coarsening, occurs at a rate that increases with temperature2, 3, so that snow albedo generally decreases as temperature increases. This increases energy absorption at the surface and atmospheric warming ensues, leading to a positive snow-albedo feedback. Here we use passive microwave satellite data and model outputs to show that this feedback is inhibited by small increases in precipitation. This is explained by the fact that grain coarsening in Antarctica is more sensitive to the deposition of small grains on the surface than previously assumed. We deduce that projected future increases in precipitation4 can increase snow albedo by 0.4% on average during the twenty-first century and hence overcompensate the expected albedo decrease owing to warming (0.3% for 3°C). Albedo-change projections in the Coupled Model Intercomparison Projects 3 and 5 do not reach a consensus on the sign and amplitude of this compensation, showing the need for a finer representation of the impact of precipitation on albedo in Antarctica.